Crossed-field electric discharge device



April 18, 1967 A S CROSSED-FIELD ELECTRIC DISCHARGE DEVICE Original Filed April 27, 1961 6 Sheets-Sheet 1 INVENTOR JAMES E. STAATS BY 5 J-lZg'y A TTYS.

CROSSED-FIELD ELECTRIC DISCHARGE DEVICE Original Filed April 27, 1961 6 Sheets-Sheet 2 April 18, 1967 J. E. STAATS 3,315,121

CROSSED-FIELD ELECTRIC DISCHARGE DEVICE Original Filed April 27, 1961 6 Sheets-Sheet 5 I050- PLAIN CA 711005" F|G.7.

o Q I I a 900 a? E 8. e50 f ELECTRON l- DIRECTION -b u 800- Q 0 I k 750- 8 l I I I I 0 I0 30 50 7O 90 PERCENTAGE DISPLACEMENT 0F CATHODE SECTIONS RELATIVE TO ANODE SECTIONS.

April 18, 1967 J. E. STAATS 3,315,121

GROSSED-FIELD ELECTRIC DISCHARGE DEVICE Original Filed April 2'7. 1961 6 Shets-Sheet 4 FIG.9.

EL zcrnoiv DIRECTION April 18, 1967 J. E. S 'AA'TS 3,315,121

CRQSSED-FIELD ELECTRIC DISCHARGE DEVICE Original Filed April 27, 1961 6 Sheets-Sheet 5 F|G.l2.

O.I THERMAL BOUNDARY THERMAL BOUNDARY UNSTABLE April 18, 1967 J. E. STAATS 3,315,121

CROSSED-FIELD ELECTRIC DISCHARGE DEVICE Original Filed April 27, 1961 6 Sheets-Sheet 6 m FIG.|4. q 1 150k F5 is 5; I00 i w u u 0 2K 2 .50 2.1 if I 0 g a: mi k n k IO0 0 I0 20 3O 4O 1 50 6O 7O 8O 9O I00 PERCENT DISPLACEMENT OF CATHOOE SECTIONS RELATIVE TO ANODE SECTIONS MAXIMUM TOLERABLE VOLTAGE STANDING WAVE RATIO (VSWR) I I l l l I l I l O I O 20 3O 40 5O 6O 7O 8O 90 I00 PERCENT DISPLACEMENT OF CATHODE SECTIONS RELATIVE TO ANODE SECTIONS United States Patent 6) 3,315,121 CROSSED-FIELD ELECTRIC DISCHARGE DEVICE James E. Staats, Louisville, Ky., assignor to General Electric Company, a corporation of New York Continuation of application Ser. No. 105,983, Apr. 27, 1961. This application Jan. 10, 1966, Ser. No. 519,629 Claims. (Cl. 315-3951) -This application is a continuation of the copending application for United States Letters Patent of James E. Staats, Ser. No. 105,983, filed Apr. 27, 1961, for Crossed- Field Electric Discharge Device, now abandoned.

My invention relates to crossed-field electric discharge devices and pertains more particularly to multiple cavity magnetron devices adapted for microwave heating applications, such as electronic cooking.

For applications such as electronic cooking it is desirable to provide a magnetron adapted for low voltage operation, thus to obviate the need for elaborate and expensive power supply apparatus. Such power supply apparatus would add considerably to the cost of the overall equipment and would result in a selling price prohibitively high in the home appliance market and at least unattractive in the commercial cooking apparatus market.

In addition to being adapted for low voltage operation, a magnetron adapted for cooking applications must satisfy various other requirements including satisfactory opera tion within a predetermined frequency range, high efficiency, high power output, stability of operation, long life and minimum manufacturing cost for the magnetron device per so as well as for the apparatus employed therewith. v i

The operating frequency of a magnetron device is determined primarily by the construction of the vanes and the strapping rings interconnecting alternative vanes. The voltage and efiiciency of a magnetron device are determined by the ratio of the anode inner diameter to the number of anode segments or resonators and by the ratio of the cathode outer diameter to the anode inner diameter. I have found that a low voltage, highly efficient magnetron the operating temperature of can be obtained by providing a structure wherein theratio of the cathode outer diameter to the anode inner diameter is substantially larger than that ordinarily expected to be used in an operative device. Accordingly, my invention contemplates the provision of a new and improved magnetron device wherein the anode structure is suchthat the ratio of the cathode outer diameter to the anode inner diameter ishigher than usual, whereby the device is adapted for low voltage and highly efficient operation.

' Stability of magnetron operation refers to the maintenance of a constant frequency output regardless of load impedance variations and the avoidance of harmonic radiation or modes which are characterized by different frequencies. Desired stability and efficiency are attained in part with my improved device by the provision of a new and improved output circuit adapted for conducting a relatively large current at a relatively low voltage, transforming such low voltage to a lower voltage required by a load, avoiding coupling reactance into the circuit at the operating frequency and attenuating spurious radiation. Additionally, my device includes a new and improved output end cap construction adapted for serving as a radio frequency by-pass capacitor which further reduces harmonic radiation.

An improved arrangement including an internally located capacitor associated with the anode construction serves to compensate for any tendency toward unbalance of the cathode and, thus, minimizes undesirable radio frequency power flow out of the cathode and filament leads. Additionally, the capacitor serves to compensate for any unbalance introduced by the output circuit. In my improved structure the zcapaci-ty of the output circuit acts through the inductance of the output leads while the balance capacitor acts directly on an opposite strap ring. Thus balance is not maintained for frequencies other than pi mode and the capacitor operates beneficially to cause unfavorable conditions for oscillations of the device in other than the pi mode, whereby operating efficiency is further enhanced.

Magnetron operating life is generally dependent upon cathode life and cathode life is generally reduced substantially by the destructive effects of back heating of the cathode in magnetrons where close anode-to-cathode spacing is encountered. Additionally, in electronic cooking applications the load impedance is generally difiierent depending, for example, on the sizes and materials of the items being heated. Further, the load impedance generally varies in accordance with varying stages of heating or cooking. These load impedance changes tend to cause variations in the back heating effect and thus can vary the cathode. Thus, variations of the load impedance with resultant variations in the degree of back heating can cause over-heating of the cathode and shortened cathode life. Further, the variations can cause cathode temperature reductions which can result in undesired radio frequency power reduction or increased heater power requirements. Consequently, the provision of means for rendering cathode temperature less sensitive to variations and differences in load impedances is desirable. Also, it is often desirable to increase the maximum voltage standing wave ratio which is attainable before the magnetron becomes unstable and ceases to oscillate. Expressed in another manner, it is often desirable to increase or maximize the tolerable load on the magnetron.

My invention contemplates provision of new and improved means effective for enabling unusually close anodeto-cathode spacing while reducing substantially back heating of the cathode. Further, my invention contemplates new and improved means effective for minimizing variations in cathode back heating in magnetron devices supplying power to varying and different load impedances and for maximizing the tolerable load on the magnetron devices.

Accordingly, a primary object of my invention is to provide a new and improved magnetron device of a type particularly adapted for microwave heating applications such as electronic cooking.

Another object of my invention is to provide a new and improved magnetron device particularly adapted for low voltage, high power output operation.

Another object of my invention is to provide a new and improved magnetron device including new and improved self-contained means for suppressing spurious radiation.

Another object of my invention is to provide a new and improved magnetron device including new and improved means for increasing operating stability and efiiciency of the device.

Another object of my invention is to provide new and improved means for controlling cathode back heating in magnetron devices and insuring greater uniformity of power output.

Another object of my invention is to provide a new and improved cathode construction adapted for increasing efficiency, power output, stability and maximum useful load toleration of a magnetron.

Another object of my invention is to provide a new and improved cathode adapted for enabling the construction of a magnetron with smaller cathode and anode diameters, thereby to provide an unusually large ratio of the cathode outer diameter to the anode inner diameter, thus, affording increased operating efficiency and power output.

Another object of my invention is to provide a new and improved magnetron structure which enables the production manufacture of magnetron devices characterized by substantial uniformity of power output when operated under like conditions.

Further objects and advantages of my invention will become apparent as the following description proceeds and the features of novelty which characterize my invention will be pointed out with particularity in the claims annexed to and forming a part of this specification.

In carrying out the objects of my invention I provide an improved multiple cavity resonator magnetron wherein low voltage operation is obtained through the employment of an unconventionally large cathode-to-anode diameter ratio. The device includes an improved output circuit providing a plurality of discrete coupling points, between adjacent pairs of which points are located a predetermined plurality of resonator cavities and which output circuit is adapted for improving operating efficiency and suppressing undesired modes. The output seal of the device has a predetermined electrical relationship to the output circuit to avoid destructive high electrical potentials at the seal. Additionally, the end cap of the device through which the output seal is made is constructed to serve as a radio frequency by-pass capacitor effective for reducing harmonic radiation. A built-in capacitor construction comprising a conductive ring supported in predeterminedly spaced relation to the anode on the side thereof opposite the output circuit is provided to compensate for any tendency toward electrical unbalance resulting from the cathode or output circuit and to unbalance the fields in the device for modes other than the pi mode. Additionally, the cathode includes periodic radial protrusions predeterminedly located relative to the anode segments and, thus, is adapted for improving magnetron operating efficiency, power output, and stability. My improved device including the periodic cathode is also effective for maximizing load toleration and for minimizing back heating and variations in back heating when power is being supplied to varying and different load impedances.

For a better understanding of my invention reference may be had to the accompanying drawings in which:

FIGURE 1 is a sectional view of magnetron apparatus constructed in accordance with an embodiment of my invention;

FIGURE 2 is a fragmentary plan view taken along the line 2-2 in FIGURE 1 and looking in the direction of the arrows;

FIGURE 3 is an enlarged fragmentary'sectionalized view illustrating particular features of the embodiment of FIGURE 1;

FIGURE 4 is a perspective view of the cathode mounting clamp;

FIGURE 5 is a fragmentary perspective view illustrating the air-cooling construction of my invention;

FIGURE 6 is a fragmentary and somewhat schematic view of a modified form of my invention;

FIGURE 7 is an enlarged fragmentary illustration of a portion of FIGURE 6;

FIGURE 8 is a characteristic curve showing the heating of a cathode constructed according to my invention as compared with the heating of a conventional straight cylindrical cathode;

FIGURE 9 is a schematic illustration of the positional relation of the anode segments and cathode protrusions in the structure of FIGURES 6 and 7;

FIGURE 10 is a schematic illustration of the method whereby the positional relation of the anode segments and the cathode protrusions can be determined for optimum performance;

FIGURE 11 is an enlarged fragmentary perspective illustration of a modified form of the cathode structure illustrated in FIGURES 6 and 7;

FIGURE 12 is a simplified Rieke diagram on which are charted variations in the performance. of a prior art type of magnetron resulting from changes in the load impedance;

FIGURE 13 is a simplified Rieke diagram on which are charted variations in the performance of my improved structure resulting from variations in the load impedance;

FIGURE 14 is a curve plotting the difference in cathode temperature swing with different percentages of relative displacement between the periodic cathode and the anode sections; and

FIGURE 15 is a curve plotting the'maximum attainable voltage standing wave ratio with different percentages of relative displacement between the periodic cathode and the anode sections.

Referring to FIGURE 1, there is shown an embodiment of my invention including a magnetron device and 'a magnetic circuit generally designated 1 and 2, respectively. The magnetron 1 comprises several general subassemblies including an envelope 3, an anode structure 4, an output circuit 5 and a cathode assembly 6.

The magnetron envelope 3 includes a tubular wall member 7 formed preferably of copper or any similar high thermal conductivity non-magnetic metal. Suitably sealed to the ends of the wall member 7, as by heliarc welding, are upper and lower end caps 10 and 11, respectively, which extend completely across the open ends of the wall member 7. The end caps 10 and 11 are formed preferably of steel and are shaped to provide centrally disposed protruding cup-like portions 12 which are of substantially less diameter than the outer diameters of the end caps. Additionally, the end caps are formed to include cylindrical rims or lip portions 13 at which the seals to the ends of the member 7 are effected. Sealed to the lower end cap 11 is an exhaust tubulation 15 through which the envelope is adapted for being evacuated to a high degree. Following such evacuation the end of the tubulation is ordinarily pinched off and sealed in a conventional manner.

Secured to the external surface of the envelope member 7, as by brazing, is a stacked array of heat-dissipating fins 16 which are also formed preferably of copper or any other metal of suitable high heat conductivity. As seen in FIGURE 1, the fins 16 are mutually spaced. Additionally, as seen in FIGURE 2 the fins are generally rectangular in plan view. The purpose for this configuration will be brought out in detail hereinafter with regard to the description of the magnetic circuit 2.

Contained in the envelope 3 and mounted on the inner wall of the tubular member 7 is the anode structure 4. The anode structure 4 comprises a plurality of radially extending segments or vanes 20 which, as seen in FIG- URE 2, define an axially extending central space in the device and are angularly spaced, thereby to define a plurality of circumferentially arranged cavity resonators opening into the central space. The vanes 20 are generally I-I-shaped to provide a predetermined capacity between the immediately adjacent vanes comprising each resonator. Additionally, the H-shape provides an inner end portion or tip 21 of predetermined height for cooperating with the cathode 6 and an outer portion 22 of sufficient length to. provide a satisfactory braze to the inner surface of the wall member 7. Further, the heights of the vanes 20 relative to the length of the tubular member 7 are such as to provide substantial end spaces disposed between-the anode structure 4 and each of the end caps 10 and 11.

' Disposed in the end spaces between the end caps 10 and 11 and anode structure 4 are upper and lower internal pole plates 23 and 24, respectively. The pole plates 23 and'24 are formed of a suitable high magnetic permeability material, such as iron, and are each provided with a copper plating to afford a satisfactory radio frequency conduction surface. Additionally, the pole plates 23 and 24 are shaped to include planarrim portions 25 and central dished portions 26. The rim portions 25 engage theplanar outer portions of the end caps 10 and 11, respectively, and have the peripheries thereof secured in suitable steps formed in' the wall member 7. The central portions 26 of the pole plates 23 and 24 extend toward each other and toward the central space defined by the inner ends 21 on the anode vanes 20. Thus, the pole plates 23 and 24 are adapted for predisposing and concent-rating in the mentioned central space in the anode structure 4 the flux lines of a magnetic field provided by the mentioned magnetic circuit apparatus 2.

The vanes 20 are preferably thirty or more in number and constitute two sets of oppositely positioned vanes which are alternately disposed. The vanes 20 of the first set are positioned in the manner of the vane 20 on the right hand side in FIGURES l and 3 and so as to locate an inwardly disposed step 30 on the upper edge and an outwardly disposed step 31 on the lower edge thereof. The vanes 20 of the second set are positioned in the manner of the vane 20 on the left hand side of FIGURES 1 and 3 and so as to locate an outwardly disposed step 31 on the upper edge and an inwardly disposed step 30 on the lower edge thereof. Conductively secured to and electrically interconnecting the steps 30 on the upper edges of the first-mentioned set of vanes 20 and spaced from the upper edges of the vanes 20 of the second set is a circular conductive strapping element or strap 32. Similarly interconnecting the steps 31 on the upper edges of the second-mentioned set of vanes 20 is a circular conductive strapping element or strap 33 which is disposed concentrically about the strap 32. This arrangement is repeated on the lower side of the anode structure 4 but with a lower inner strap 32 interconnecting the same set of vanes 20 as the upper outer strap 33 and with a lower strap 33 interconnecting the same set of vanes 20 as the upper inner strap 32. As is well known in the art, this form of strapping arrangement is provided in magnetrons for eliminating spurious modes of oscillation and thus enabling such devices to oscillate in the desired pi mode and to generate substantially increased amounts of power with substantially increased elficiency. The upper inner strap .32 also comprises an element in my improved output circuit 5 which shall be described in greater detail hereinafter. a

Disposed centrally in the space defined by the inner end portions 21 of the anode vanes 20 is the above-mentioned cathode assembly 6. The cathode assembly 6 is of the indirectly-heated type and is supported on the lower pole plate 24. As best seen in FIGURE 3, the lower pole plate 24 includes an enlarged central aperture 34 and a plurality of surrounding circumferentially spaced smaller apertures 35. Extending through the central aperture 34 is a conductive center post 36. To the upper end of the center post 36 is conductively connected one end of a filamentary heater 37 which spirals downwardly about .the center post 36 in laterally-spaced relation thereto.

Fitted on the center post 36 on each side of the spiral portion of the heater is a metal foil disk 40. Welded to rims formed on the disks 40 are the ends of a foil cathode sleeve 41. The cathode sleeve 41 can be formed either of a highly emissive material or, alternatively, can be provided with a sintered porous coating impregnated with a suitable emissive oxide material. In this arrangement the end disks 40 and the cathode sleeve 41 are preferably only approximately 6 mils thick and, thus, the sleeve 41 is adapted for fast heating upon energization of the filament 37. Additionally, due to the thinness of the end disks 40 no substantial heat loss from the sleeve 41 to the center post 36 by conductivity through the disks 40 is experienced. Thus, a desirable, more uniform temperature along the length of the sleeve 41 is obtained. Further, because of their thinness the end disks 40 are highly flexible. As a result, any thermal expansion experienced by the end disks 40 is not effective in distorting the oathode sleeve 41; and, thus, the concentricity of the cathode sleeve 41 in the central space in the anode structure 4 and circumferentially uniform interelectrode spacing are maintained. Variations in the interelectrode spacing between the cathode sleeve 41 and the anode structure 4 can be highly detrimental to the operation of the tube 1.

Also, on the center post 36 and in positions outwardly spaced from the disks and the sleeve 41 are provided end shields 42. The end shields 42 are effective for avoiding undue heat losses by radiation from the ends of the cathode assembly. This contributes to greater thermal efficiency and more uniform heating of the cathode sleeve 41. Additionally, the end shields 42 are formed prefer ably of low emissivity material and inasmuch as the end shields 42 are not directly connected to the cathode sleeve 41 but instead are longitudinally spaced therefrom, the temperatures of the end shields 42 are maintained below that of the cathode sleeve 41 which serves to prevent undesired electron emission from the shields 42.

The lower end of the filament 37 extends through an insulative sleeve 43 which, in turn, extends through the lower one of the end shields 42. The lower end shield 42 and a similar metal disk 44 cooperate with a pair of stepped insulative washers 45 in providing a clamping arrangement for supporting lthe cathode assembly. Specifically, the insulative washers 45 are each disposed on the opposite sides of the lower pole plate 24 and include central portions extending into the central aperture 34 in the pole plate 24. The washers 45 are longitudinally spaced and are held in clamping relation with the pole plate 24 by a spring clamp 46 which is preferably formedof .Inconel and which engages the underside of the lower pole plate 24. The spring clamp 46 is also centrally apertured and has inserted therein an insulative collar 50 which fits over the lower end of the center post 36 and against the underside of thespring clamp 46. A nut 51 threadedly engages the lower end of thecenter post 36 and bears against theinsulative collar 50. As better seen in FIGURE 4, ,the spring clamp 46 is generally U shaped and the legs thereof are formed with sharpened or pointed edges 52. In the structure just described, tightening of the nut 51 causes theedges 52 to dig into the copper plating on the lower side of the pole plate 24 and to draw the lower shield 42 into tight engagement with the upper surface of the pole plate 24 through the insulative washers 45. Thus, the clamping and cathode assembly 6 are secured relative to the pole plate 24 and the cathode assembly 6 can be readily adjusted concentrically relative to the anode structure 4'. Additionally, the spring clamp 46 is formed of suflicient size and area so as to be. self-cooling in the device, thereby to avoid annealing of the spring clamp 46 which would render it ineffective for its intended purpose.

Conductively secured to the lower end of the center post 36 is a lead 53 which extends in sealed insulative relation through a concentric seal 54 carried by the lower end cap 12 shown in FIGURE 1. The lower end of the filament 37 is connected to the disk 44, through, which the center post 36 extends in insulated spaced relation and conductively connected to the disk 44 is another lead '55 which extends in sealed insulated relation through a concentric seal 56 carried by the lower end cap 12. Thus is provided on the exterior of the envelope 3 a pair of conductive leads 53 and 55 for completing an energizing circuit through the cathode filament 33 and for providing a cathode contact.

As discussed above, it is desirable that the described device be adapted for low voltage, efiicient operation. As also explained above, the voltage and efficiency of a magnetron are known to be determined in part by the ratio of the cathode outer diameter to the anode inner diameter. In convention-a1 magnetron construction this ratio is usually based on a cathode outer diameter to anode inner diameter ratio of N4 to N+4 or N-pi to Nf-pi, where N is the number of vanes or resonators of the anode structure.

My invention involves a cathode outer diameter to anode inner diameter ratio which is substantially larger than .that ordinarily expected to be employable in an operative magnetron. Specifically, I employ a cathode outer diameter to anode inner diameter ratio which preferably satisfies approximately the ratio of N-2 to-N +2 where N is the number of vanes or resonators of the anode structure and, also, where N is a number between about 16 and 36.

With this larger than usual ratio, I have been able to obtain desirably low voltage operation and increased efficiency. The low voltage requirement of my improved structure enables operation of the device with the relatively low 220 volt source of a home outlet and an appropriate rectifying circuit and, thus, my improved device is particularly adapted for appliance applicationssuch as electronic cooking.

While I have indicated above that an enlarged ratio of approximately N-2 to N +2,'and where N is a number between approximately 16 and 36, is preferred, it is to be understood from the foregoing that the disclosed ratio can vary somewhat from that indicated as preferable. In fact, the benefits of my invention are obtainable with a ratio of N-(1.50 to 2.5) to N+(1.50 to 2.5) and where N is a number between about 16 and 36.

With the above-discussed cathode radius to anode radius ratio I have been able to obtain magnetron operation in the desirable frequency band of 915 me. at 570 volts DC. and with maximum power output of 700 watts. Stable operation was also obtained at an applied anode to cathode voltage as low as 400 volts DC, the power output being about 200 watts. Additionally, I have been able to obtain peak power output of 2400 watts at about 1000 volts.

As seen in FIGURES 1 to 3, the output circuit of my improved device includes as an element thereof the inner upper strap 32. This output circuit 5 provides for a number of operational advantages. For example, it is adapted for conducting a relatively large current at a low voltage from the anode structure 4 to an external load and for transforming this low voltage to a relatively lower voltage required by the load. Additionally, it is adapted for avoiding the coupling-in of reactance at the desired pi mode operating frequency and to be highly reactive at all other mode frequencies, thus to cause an unbalance of electric fields for these other mode frequencies which are undesirable modes in the operation of the device. Additionally, my device is adapted for suppressing undesired modes, or attenuating spurious radiation, by effecting non-symmetrical coupling of the undesired modes and thereby reducing the electric fields of these undesired modes while improving the desired pi mode fields.

Specifically, my improved output circuit 5 comprises the inner upper strap 32 which is conductively connected to alternate anode vanes 20, a plurality of upstanding circumferentially spaced conductive connectors or rods 60, a conductive disk 61 to which the rods 60 are secured and a central conductor or rod 62. By means of a concentric ceramic-and-metal seal construction 63 the central conductor 62 extends axially through and is electrically insulated from the upper end cap 10. Externally of the envelope 3, the rod 62 comprises the inner conductor of a coaxial output line which is incorporated in the tube structure. The outer conductor of this coaxial output line comprises the outer surface of the cup-shaped portion 12 of the upper end cap 10. Provided for conducting radio frequency power from the tube to a load (not shown) is a coaxial transmission line 64 which can comprise part of the load apparatus which may be a so-called electronic cooking oven.

My described output circuit 5 affords the various abovediscussed operational advantages by providing for a plurality of discrete coupling points or connect-ions to the anode structure 4 and providing such coupling at predetermined points at which the electric fields of all modes except the pi mode will be reduced.

Specifically, it is well known that magnetrons having a relatively small number of resonators such as 6, 8 or 10 in number have satisfactorily high performance characteristics. According to my invention a substantially greater total number of resonators is provided in the anode structure and the coupling points are arranged in circumferentially spaced relation so that the number of resonators located between adjacent coupling points is relatively small, such as 6, 8 or 10. Thus, between each pair of adjacent coupling points is provided resonator structure comparable generally to the anode structures which each in total include only a relatively few resonators, such as 6, 8 or 10 and which exhibit satisfactorily high performance characteristics.

Also according to my invention, the coupling points are located such as to attenuate the electric fields of all modes except the pi mode for thus suppressing such all other modes which are undesired in the normal operation of the device. Specifically, in my improved struc ture the output circuit 5 has a low pass filter characteristic which adapts it for attenuating spurious radiations. The spurious radiations have higher frequencies than the pi mode frequency and are attenuated by the output circuit 5. In general the voltages at the several output connections to the anode circuit will be different from the voltages for all significant modes except the pi mode. Thus, at other than the pi mode, currents will tend to fiow between the points of differing potential through the several rods 60 and the disk 61, which will have the desirable effect of attenuating the spurious radiation. However, at the pi mode the voltage at all the several connections will be substantially equal and to these voltages the output circuit 5 will appear as a plurality of output circuits connected in parallel and providing a low series resistance path at the operating frequency.

The just-discussed feature of my invention may be represented mathematically as where P is the number of discrete coupling points required to obtain the mentioned operational advantages, N is the number of resonators and M is any integer which is greater than 1 but less than N 2 and which will make P a whole number.

For example, with the illustrated anode structure 4 which includes thirty resonators the number of coupling points according to my invention should be 5 or 3. In the embodiment illustrated in the drawing, 5 rods 60 are provided to make 5 discrete coupling points between the strap 32 and the center rod 62 through the disk 61 and 6 resonators are located between each adjacent pair of coupling points. Alternatively, and also according to my invention, 3 discrete coupling points could be employed with a resonator anode structure, in which case 10 resonators would be located between each pair of adjacent coupling points.

It is to be understood from the foregoing that my invention is not limited to 30 resonator magnetrons but is equally applicable to magnetrons including lesser or greater numbers of resonators, provided the above-noted formulation is adhered to in determining the number of discrete coupling points to be employed in making connections to the anode structure. It is to be understood further, however, that desirable low voltage operation and increased efficiency are advantages accruing when a large number of resonators are employed and a small ratio of anode radius to the number of resonators results. Thus, an anode structure including approximately 30 or more resonators is preferable.

Additionally, it is to be understood that while I have shown and described the coupling points as located on the inner strap 32, this is merely a mechanical manufacturing expedient inasmuch as it facilitates assembly by enabling insertion of the individual rods through appropriate apertures 65 in the upper pole plate 23 and insertion and brazing of the rod ends in suitably drilled holes in the inner strap 32. The advantages of my invention can be obtained so long as a number of output coupling points between the output circuit and the resonator structure are provided which satisfy the abovenoted formulation. Alternatively, the connections could be made to the outer strap or directly to the edges of appropriately located anode vanes of like polarity. The advantages of my invention can also be obtained with the use of a plurality of discrete inductive coupling loops or a plurality of discrete capacitive couplings cooperating with the resonator structure at spaced points which satisfy the above-noted formulation regarding the number of coupling points required.

Thus, it will be seen from the foregoing that I have provided a novel magnetron structure adapted for improved magnetron performance by internally suppressing undesired modes using a plurality of discrete couplings which introduce non-symmetrical coupling for the undesired modes and, thus, reducing electric fields of the undesired modes while improving the pi mode fields. Additionally, my invention improves magnetron performance by suppressing undesired modes through the employment of a plurality of discrete couplings which introduce a reactance to cause unbalance of the electric fields of the undesired modes while leaving the pi mode balanced.

In my described improved output circuit the common output lead 62 acts as a transformer which decreases the voltage in the transmission line 64 to the load. Further, the output circuit 5 is constructed to provide the desired transformer action in a manner such that the transmission line 64 connected to the output circuit 5 provides the proper impedance for the magnetron. Specifically, the output circuit 5 and the output seal 63 areconstructed and relatively located according to my invention so that the seal 63 is located electrical wave length away from the resonator terminals, or discrete coupling points between the rods 60 .and the straps 32, in order thereby to locate the output seal 63 at a point of minimum voltage under matched load conditions. Furthermore, even if the load should be mismatched, a lower voltage is obtained at .the A wave length point than within the A wave length point. Thus, according to a feature of my invention,. the output seal 63 is located at or at least immediately adjacent to a point of the transformer section which is A electrical wave length from the resonator terminals so that dielectric losses and stresses at the seal 63 are minimized. As a result, a relatively small lead seal 63 can be employed to carry a large amount of power output without electrical breakdown. Additionally, the smaller seal 63 reduces the cost of manufacture and reduces the sealing area which minimizes the danger of leakage.

The unusually large cathode outer diameter to anode inner diameter ratio discussed above results in unusually close interelectrode spacing between the anode and cathode which, in turn, introduces a large capacitance in the radio frequency circuit. This, together with the inductance and capacitance of the cathode mount, results in considerable total capacitance in the radio frequency circuit. Thus, when the radio frequency voltage on the cathode is not zero the cathode would ordinarily tend to be unbalanced and there would be a tendency for radio frequency power to flow out the cathode lead. This power can result in radio frequency interference with the operation of other electronic apparatus as well as possible health hazards. Additionally, it can cause undesired inductive heating of adjacent elements and damage thereto. Also, the field patterns would tend to be unbalanced with resultant undesired reduction in tube operating efliciency. Further, connection of the output circuit to one side of the inductance anode structure tends to result in an unbalanced output. Thus, should the loadv in the output circuit 5 be characterized by a large capacitive reactance, the cathode would become further unbalanced. These unbalanced conditions are undesirable and are avoided according to my invention by the provision of a balance capacitor cooperating with the side of the inductance opposite the output circuit 5.

Specifically, and as seen in FIGURES 1 and 3, I have provided a balance capacitor in the form of a conductive ring 66 which has a predetermined surface area, is se curely mounted on the lower pole plate 24 and is located opposite the lower strap 32 which is connected to the anode vanes 20 of polarity opposite to that of those conductively connected to the output rods 60. With this arrangement cathode balance is maintained even when the load includes a large capacitive reactance. Additionally, the balance capacitor is adjusted by predetermined spacing relative to the lower strap 32 to provide a capacitance on the lower side of the resonator structure generally equal to the capacitance in the output circuit 5 on the other side and thereby is effective to compensate for any unbalance introduced by the output circuit 5. Specifically, the capacitor 66 cooperates with the lower strap 32 to provide a lumped capacitance which is effective at the pi mode frequency to compensate for the unbalance introduced by the output circuit 5. At all other frequencies, or frequencies other than the pi mode frequency, the output circuit 5 continues unbalanced, with the result that the field patterns for these undesired modes are unbalanced. Such unbalance has the beneficial effect of causing unfavorable conditions for oscillation in the modes or frequencies other than the pi mode, and, thus, radiation from the device at these other frequencies is suppressed.

In my improved device, the upper output end cap 10 serves as an additional low pass filter which reduces harmonic radiation. Specifically, the end cap 10 is located approximately electrical wave length from the resonator terminals at the operating frequency. Thus, at the second harmonic and other higher frequencies the end cap acts as a radio frequency by-pass capacitor. Specifically, the end cap 10 is of substantial area relative to the elements comprising the portion of the output circuit including the rods 60, the disk 61 and the central conductor 62. Thus, substantial capacitance is afforded between these elements, and at the second harmonic and other higher frequencies the structure acts as a radio frequency shunt capacitance with the result that such higher frequency energy is not transmitted to the load.

Illustrated in FIGURES 1, 2 and 5 is a magnetic circuit arrangement 2 particularly useful with the improved magnetron device 1 of the present invention. As best seen in FIGURE 1, the construction of the end caps 10 and 11 is such as to provide central dished portions of'reduced diameter. This construction allows the use of a relatively small electromagnetic coil 70 having the dished portion 1-2 of a respective end cap extending reentrantly therein, at either end of the device. Additionally, this arrangement enables the employment of relatively inexpensive coils 70 and yet is effective for providing a substantially uniform concentrated magneticfield ex-tending'coaxially through the magnetron.

The coils 70 fit over iron cylinders 71 which extend inwardly to and engage the planar portions of the end caps 10 and 11 and, thus, bring the magnetic flux into close association with the pole plates 23 and 24. This arrangement results in more concentrated flux density distribution in the interelectrode region between the pole plates 23 and 24.

Provided for cooperating with the cylinders 71 to com plete the required return magnetic path is a box-like construction generally designated 72. The cylinders 71 and the box-like construction 72 are formed of any suitable high magnetic permability material, and the box-like construction 72 is open ended. Additionally, the box-like construction 72 is suitably apertured. at the upper and lower sides for extension therethrough of the cathode and filament lead connections at the lower end of the device 1 and the output line 64 at the upper end of the device 1. The construction 72 carries rings 73 formed of magnetic permeability also and suitably fastened to the upper and lower sides of the construction 72. Each of the rings 73 include-s cylindrical portions tightly fitted in the coil cylinders 71 and which are effective for increasing the available magnetic flux path in the vicinity of the outer portions of the cylinders 71.

As seen in FIGURE 1, the box-like construction 72 is preferably in two separate parts, namely, upper and lower channel-like-sections 78 of identical construction and secured by fasteners 79. This construction enables the magnetron 1 to be readily and securely mounted and supported in the box-like construction 72. Specifically, the upper and lower sections 78 can be separated. Thereafter the magnetron 1 can be inserted to rest on the lower cylinder 71, following which the upper section 78 can be placed with the upper cylinder 71 engaging the upper end cap 10. Thereafter, suitable fastening means, such as the bolts 79, interconnecting the sides of the upper and lower sections 78 can be tightened whereby the magnetron 1 will be securely mounted in the box-like construction 72 with the tube 1 and magnet coils 70 concentrically aligned and with the cooling fins 16 appropriately positioned in the construction in the manner shown in FIG- URE 1 Suitably secured to the lower side of the box-like construction 72 is a cylindrical filament connector housing generally designated 74, and over the lower end of which is fitted a cover 75 held in place by a suitable clamping arrangement 76. The cover 75 carries a pair of terminal connectors 77 on the outer side thereof. Additionally, the cover 75 serves to support a pair of radio frequency by-pass capacitors 80 each in electrically connected relation between a filament terminal on the magnetron 1 and one of the terminals carried by the cover 75. The bypass capacitors arc of the coaxial type and have the outer conduct-or thereof connected to ground through the cover 75. Thus, the capacitors 80 are effective for conducting the cathode DC. current and an A.C. cathode heater current and for shunting to ground any radio frequency leakage from the cathode leads.

As indicated above, the box-like construction 72 serves as the return magnetic path between the electromagnetic coils 70. As perhaps best seen in FIGURE 5, the construction 72 is further adapted for serving as a cooling duct for the magnetron 1 disposed therein. The member 72 is adapted for having a cooling fluid, such as air, directed therethrough, as by means of a suitable air fan 81 mounted at one end thereof. Thus, a cooling fluid may be directed against the magnetron 1 to dissipate heat emanating therefrom. This heat dissipation is enhanced by the provision of the stacked array of parallel cooling fins 16 secured, as by brazing, on the outer wall of the envelope 3. The fins 16 extend in planes parallel to the direction of the coolant flow through the construction 72 and the spaces between the fins 16 are unobstructed to avoid any substantial impedance of coolant flow therepast. Additionally, and as best seen in FIGURE 2, the fins 16 are generally rectangular in plan view and the transverse dimension thereof in the duct closely approaches the width of the duct. This serves to maximize the heat dissipating surfaces presented to a coolant flowing in the duct. If desired, the width of the fins 16, or in other words the longitudinal dimension of the fins 16 when disposed in the duct, can be increased to increase the available heat dissipating surface area.

It is also to be appreciated from the foregoing, that the walls of the duct or box-like construction 72 serve also in providing a substantial heat-dissipating surface area which minimizes the cooling requirements placed, for example, on the cooling fan 81 as well as the required size of the fins 16 for a given heat-dissipating capacity,

all of which serve to minimize the overall space requirements of the apparatus. Further details of the construction of the magnetic circuit 2 described above are set forth in and are claimed in my copending application, Ser. No. 283,355, filed May 27, 1963, which constitutes a continuation-in-part of the prior application, Ser. No. 105,983.

Illustrated in FIGURE 6 is a form of my invention which is particularly effective in minimizing undesirable back heating of the cathode, minimizing variations in any back heating that may tend to result from different loads or variations in load values placed on the magnetron and in maximizing load toleration, or the maximum VSWR attainable before the device will become unstable and cease oscillating.

In the operation of a magnetron, and according to classical magnetron theory, a portion of the generated power is utilized to bunch electrons in providing rotating spokes of electrons which serve to transfer energy to the resonant cavities in the anode structure. The electrons which are emitted from the cathode toward the anode and which are successful in contributing positively to the bunching operation are generally termed favorable electrons. Many other electrons, however, are emitted from the cathode and are returned thereto under the influence of the magnetic field relatively quickly and without contributing appreciably to the mentioned bunching process. Additionally, these electrons absorb energy from the radio frequency electric field and are usually returned to the cathode with sufficient accelerating force and bombarding effect to contribute substantially to the heating of the cathode. This condition is referred to in the art as back heating and is generally undesirable because it often results in uncontrolled heating and emissivity of the cathode and, thus, reduces the power level obtainable for a given operating temperature. Additionally, the elec trons which cause the back heating and fail to contrib 'ute to the generation of radio frequency power are generally termed unfavorable electrons. The described un' desirable condition resulting from the effects of the unfavorable electrons is particularly evident and causes substantial temperature variations when the impedance or phase of the load varies. For example, in a microwave heating application differences in position, kinds, sizes and temperatures of the items being heated in an oven being energized by the magnetron can have different loading effects on the magnetron with resultant substantial effects on cathode heating. The unusually small anode to cathode spacing of my improved device, as described above, also would ordinarily predispose it toward undesirable cathode back heating effects.

My invention contemplates improved structure adapted for assisting in the bunching of the electrons, enhancing the emission of favorable electrons and suppressing the emission and undesirable effects of unfavorable electrons, thereby to increase operating efficiency of the mag netron and to provide a substantially constant cathode temperature substantially independent of impedance or phase of the load.

Illustrated in FIGURES 6 and 7 is an embodiment of my improved structure generally designated 85 having dis posed therein my improved cathode structure generally designated 90. It is to be understood from the outset that my invention is applicable to magnetrons having any number of resonator cavities. However, advantages accrue from the use of an anode having a large number of cavities, such as the 30 cavity structure illustrated in FIG URES l and 2. Specifically, the feature of my invention about to be described in detail enables construction of a magnetron'with reduced cathode and anode diameters. This, in turn, assists in enabling reduction of the ratio of the anode radius to the number of anode cavities which, as mentioned above, affords increased operating efliciency and power output. The use of an anode having a large number of cavities, such as approximately 30, also at- 13 tributes to the attainment of the desirably reduced ratio of anode radius to the number of anode cavities.

In the embodiment of FIGURES 6 and 7 the anode structure 85 includes a plurality of radially inwardly extending anode segments 87 which define a corresponding plurality of resonator cavities 86. Additionally, the inner ends of the anode segments 87 define a central coaxially extending generally cylindrical space in which is disposed a form of my improved cathode structure generally designated 90. The cathode structure 90 comprises a generally cylindrical metal sleeve 91 including a plurality of uniform periodic, or circumferentially uniformly spaced, and longitudinally extending protrusions 92 and depressions 93. This construction is similar to a cylinder formed with a plurality of uniformly spaced splines or with a uniformly corrugated outer surface.

Preferably the cathode sleeve 91 is formed to include a protrusion 92 disposed generally opposite each of the anode segments 87 or, in other words, to include N num ber of protrusions where N equals the number of anode segments 87. Additionally, the sleeve 91 is provided with an external coating of material adapted to be emissive upon heating; and an internally disposed heating element 94 is provided for heating the sleeve'to render the coating emissive. Alternatively, the sleeve 91 on which the protrusions 92 are formed can be formed of an emissive material. In either form, the outermost surface portions, or portions immediately adjacent to the anode segments 87, are adapted to be emissive when heated.

As best seen in FIGURE 7, the described structure results in an operative combination wherein the electric field lines designated 95 are concentrated between the protruding portions 92 of the cathode 90 and the innermost ends of the anode segments 87. In the regions between the protrusions 92 or, in other words, in the regions of the depressions 93, the electric fields are relatively less'concentrated and these regions are almost field free. This periodic concentration of the electric field lines 95 results in advantages in both the bunching operation, as caused by the favorable electrons emitted from the cathode 90, and in the suppression of the undesired effects caused by unfavorable electrons.

The specific manner in which the above-described structure is effective in affording the mentioned desired results is not fully understood. However, it is currently believed that in my improved structure the most favorable electrons emanate from the surfaces of the protrusions 92. It is also believed that these electrons curve under the influence of the magnetic field into the periodically concentrated electric field lines 95 between the cathode protrusions 92 and the anode segments 87. It is believed further that the effects of the periodic field concentration are such as to improve the electron bunching or rotating spoke formation process and to contribute to the radio frequency power in a manner greater than would be obtained with a right cylindrical cathode surface. It is believed still further that the unfavorable electrons emanating from the protrusions 92 tend to curve into the almost field-free regions between the protrusions 92 and thus are prevented from being accelerated substantially in their return travel to the cathode 90. As a result, these returning electrons either do not impinge upon the cathode 90 with such force as to contribute substantially to the heating thereof or traverse small loops in the regions of the depressions and reenter the interaction space for contributing to the RF. power. Additionally, it is believed that no substantial number of favorable electrons would ordinarily originate in the regions of the cathode 90 substantially midway between the extensions of the center lines of adjacent anode segments 87. Thus, in my stnucture the disposition of these mid-regions in the depressions 93 between the protrusions 92, and where it is substantially field-free, does not subtract from the availability of favorable electrons. Also, it has a highly desirable effect 14 of suppressing unfavorable electrons which would ordinarily originate in these regions.

The above-discussed improved structure is effective for minimizing back heating whether the anode segments 87 and cathode protrusions 92 are axially aligned or relatively angularly displaced. However, I have found that the degree of back heating is adjustably controllable over a wide range of temperatures by adjustably angularly displacing or rotatively positioning the cathode protrusions 92 relative to the adjacent anode segments 87 In FIGURE 8 is plotted the cathode temperature curve of a magnetron including my improved periodic cathode 90 and with the cathode protrusions 92 in various percentages of rotation relative to the corresponding anode segments 87. Additionally, FIGURE 8 Shows comparatively the'cathode temperatures of magnetrons operated with my improved periodic cathode structure 90 and with a relatively smooth-walled or plain cathode. In obtaining these data the magnetrons were operated into a matched load at 560 volts and 3 amperes and with the cathode protrusions 92 of the periodic cathode 90 angularly displaced between 0% and of the intervane spacing. Referring to FIGURE 9, the electron beam flow is to the right, the mentioned intervane spacing constitutes the distance B between the center-lines of adjacent anode segments 87, and the distance A represents the amount of relative upstream displacement of the center line of a cathode protrusion 92 and a corresponding anode segment 87. In obtaining the data of FIGURE 8 the magnetron was operated with the cathode protrusions 92 at from 0% displacement to 100% displacement, or in other words, from positions with the center line of each protrusion 92 in axial alignment with the center line of a corresponding anode segment 87 to various positions between the center line of that segment and the center line of the next adjacent segment.

It will be seen from FIGURE 8 that at any rotative position of the cathode protrusions 92 relative to the anode segments 87 the temperature of my periodic cathode 90 is substantially lower than the temperature of a plain cathode indicated by the horizontal dash line near the top of FIGURE 8. It will be seen further from FIGURE 8 that as the protrusions 92 are rotated upstream away from 0% displacement, or in the direction of electron travel, as determined by the magnetic field direction, the cathode temperature decreases until a minimum is reached between about 20% and 35%. Then temperature increases until at about 45% it corresponds with the temperature at 0% displacement. With further displacement the temperature tends to increase until about 70% displacement, beyond which it tends to decrease again. Thus, at 0% displacement, the periodic cathode 90 operates at a substantially lower temperature than a plain cathode and this temperature can be lowered still further by displacing the cathode protrusions 92 anywhere between 0% and approximately 45%. At 45% the temperature approximates that obtained at 0% even though it is still lower than that obtained with a plain cathode. Thus, from the standpoint of reduced cathode temperature a relative displacement of the cathode protrusions 92 and anode segments 87 or a ratio of A/B of between 0% and approximately 45% is considered advantageous. However, as seen in FIGURE 8 the displacement which will afford the optimum from the standpoint of reduced cathode temperature and minimum variance of temperature with load variance is approximately 15% to 35% as seen in FIGURE 14. However, a range of displacement OfxbEtWCCIl approximately 10% and approximately 40% provides a temperature range which is adequately low for low power magnetron applications. This displacement percentage will probably vary somewhat with the tube structure, load and operating voltage and current. However, it is believed that optimum displacement can be determined empirically by following the foregoing teaching.

It is also believed that optimum phase relationship, or

angular relative displacement of the cathode protrusions 92 and anode segments 87 is obtainable through use of the following mathematical representation:

where, as schematically illustrated in FIGURE 10, R is the optimum distance of rotation of the center lines of the cathode protrusions 92 from the center lines of the anode segments 87 in the direction of electron flow, as determined by the magnetic field direction, y is the minimum interelectrode space, or radial distance between the anode and cathode protrusions and X is the intravane distance, i.e., the distance between the centerlines of adjacent anode segments 87. Under these conditions, and as seen in FIGURE 10, electrons which travel a distance of 2..5X from the cathode to the anode will generally cross the electric field lines 95 at right angles, a condition required for favorable interaction.

For low voltage magnetrons, and as indicated above with reference to FIGURE 8, I have found that the optimum R is approximately to of X, butthe range of 15% of X to 35% of X is generally satisfactory.

However, for certain applications, such as operation with no heater power, other phase positions may be utilized. For example, if R=70% of X, a maximum cathode temperature is approached. This phenomena can be.

effectively employed to reduce the heater power requirements or, possibly, to obviate the need for such power.

As previously mentioned, operation of my improved structure is dependent upon the direction of electron fiow which is determined by the direction of the magnetic field. Thus, if one has adjusted the phase angle so that electrons cross the electric field at right angles, a reversal of the magnetic field, such as will cause electrons to cross the electric field at other than right angles, can disrupt the advantageous operation of my device.

It will be seen from the foregoing that my improved structure would be equally satisfactorily operative if the depressions 93 were devoid of emissive material. Thus, in FIGURE 11 I have illustrated a modified form of my invention which includes a cathode sleeve 96 which can be identical to the sleeve 91 of FIGURE 7 but which includes stripes of emissive material 97 located on only circnmferentially spaced protrusions 98. The surface portion of the cathode intermediate the protrusions 98, or, in other words, the surfaces of depressions 99, are coated with stripes of non-emissive material designated 100. Thus, emission of electrons which ordinarily would be classified as unfavorable because they fail to contribute substantially to the radio frequency power generation is minimized with resultant desirable reduction in back heating.

A further important functional advantage of the described novel magnetron structure, including a periodic cathode'having the periodic emissive sections thereof predeterminedly displaced relative to corresponding anode segments, is attributable to a reversal of the normal change of cathode temperature with load variation. This may be understood by first considering the state of the prior art to this invention, as exemplified by the report of W. Schmidt, Philips Research Reports, vol. 22, No. 3, p. 89, and particularly FIGURE 4 of that report. FIGURE 4 of the mentioned report illustrates what is generally known in the art as a Rieke diagram and on which are charted variations in tube performance resulting from changes in load impedance.

FIGURE 12 of the present application is a simplified illustration of a Rieke diagram and on which is charted variations in tube performance resulting from changes in load impedance in the operation of a prior art type of magnetron which did not include the periodic cathode structure of the present invention. The power and frequency contours which are normally shown on a Rieke diagram have been omitted from FIGURE 12 for purposes of clarity. The heavy solid lines shown in FIGURE 12 constitute cathode temperature contours and are interpositioned between a first operational region designated Tube Unstable and a second operational region designated Thermal Boundary, and operation of the mentioned prior art type of magnetron is generally unsatisfactory in either of these regions.

On the Rieke diagram, and as seen in FIGURE 12, the Tube Unstable region is in the area of heavy loading and is generally referred to as the Sink because all frequency contours converge thereat. When operated in this region, a magnetron becomes unstable and finally ceases to operate. In the opposite direction from the center of the diagram, or toward the lighter loads, the input impedance of the tube is found to decrease. This region is generally referred to as the Opposite Sink region and in this region more current is drawn and, also, the cathode temperature is found to increase due to increasing back bombardment. As is well known in the art, an increase in the cathode temperature leads to more rapid evaporation of the active cathode material, and thus reduces tube life. In some cases a still more serious effect may result since the current and temperature are both increased and may each act to reinforce the other, leading to a so-called run-away condition which will result in such overheating as to destroy the tube unless power is immediately disconnected. Thus, a certain region of the Opposite Sink area constitutes, as indicated on FIGURE 12, a thermal boundary and a limit must be generally set on the light load to avoid operation in the thermal boundary area.

In the operation of the above-discussed prior art type of magnetron the operation is restricted to about a 4 to l VSWR by the sink or tube unstable region and about a 5 to 1 VSWR by the opposite sink thermal boundary region, and all loads must be adjusted to lie between these limits.

It has been found that with the presently disclosed improved periodic cathode structure and with appropriate anode and cathode dimensions and appropriate predetermined rotational disposition of the periodic emissive sections of the cathode relative to the anode segments, and specifically in the range of about 25% to 35%, the variation of cathode temperature with load is reversed compared to the prior art case just reviewed and illustrated in FIGURE 12. This temperature reversal can be seen in FIGURE 13 which illustrates a Rieke diagram on which is charted the performance of a magnetron constructed in accordance with my present invention. The diagram of FIGURE 13 has been simplified in the same manner as the diagram of FIGURE 12 and illustrates by solid lines the cathode temperature contours and by shaded areas the tube unstable and thermal boundary regions. As seen in FIGURE 13 the variation of temperature with load is reversed compared to the operation illustrated in FIGURE 12. That is to say, the temperature of the cathode increases toward the bottom side of the diagram, or in the direction of heavier loads, such that the thermal boundary region generally coincides with the tube unstable, or sink, region. This leaves the whole of the opposite sink region of the Rieke diagram available for the load impedances, as well as the sink region out to about a 4 to 1 VSWR. In applications such as microwave cooking, wherein an extreme range of loads is generally encountered, this additional allowable region of the Rieke diagram is extremely valuable. Furthermore, the specific rotational displacement of the cathode emissive sections relative to corresponding anode segments, while reversing the direction of temperature change does not reverse the direction of input impedance change, so that in either direction from matched load the impedance change and the temperature change are of opposite signs; thus they cannot reinforce each other, and the previous danger of a runaway effect is eliminated.

For the purpose of illustrating further the above-discussed advantage of the present invention, there is illustrat'ed in FIGURE 14 a plot of temperature swing, or the difference in cathode operating temperature, between opposite sink and sink points at a 2.2 .to 1 VSWR, reckoned positive when the opposite sink point is at the higher temperature, against percentage displacement of the cathode. The data of FIGURE 14 were obtained from a tube constructed according to my invention and in which the cathode was selectively rotatable to difierent positions during operation. The existence of the desired region of negative values between 15% and 45% displacement is clearly seen from FIGURE 14, with optimum values between 25% and 35%. Further, it has been verified that when the variation is negative across the 2.2 to 1 VSWR, it continues to be negative out to a 10 to 1 or higher VSWR which is another highly advantageous feature of the present invention when applied to microwave heating applications where an extreme range of loads is generally encountered.

A still further advantage of the present invention arises from the influence of the relative displacement of the periodic cathode sections and the corresponding anode sections on the position of the sink, tube unstable, region discussed above. FIGURE 15 shows a plot of the maximum VSWR attainable without stopping oscillation, against percentage of displacement of the periodic cathode sections relative to corresponding anode sections. The data for this curve were obtained from the same rotatable cathode tube employed for obtaining the data of FIG- URE 14. It is seen from FIGURE 15 that with relative cathode displacement in the range between and 15%, the maximum tolerable load is less than a 3 to 1 VSWR, which can be an undesirable restriction. However, it is also seen from FIGURE 15 that with further relative displacement of the periodic cathode sections and anode segments the tolerable load increases rapidly, reaching a 4 to 1 VSWR at 30% to 35% displacement. Beyond this point the thermal boundary referred to above and illustrated in FIGURE 14 is reached. By combining the requirements of a rotative displacement of about 25% to 35% for obtaining the desired negative temperature variation described and referenced in FIGURE 13 and the requirement of a more than 30% displacement for desirably increasing the maximum useful load toleration, a rotation of about 30% to about 35 is obtained which is effective for providing an optimum combination of desirable tube characteristics. Thus, a displacement range of between about 30% and 35 may be considered the most advantageous.

I have also found that the operation of my improved structure is enhanced by predeterminedly dimensioning the cathode protrusions 92. Specifically, it has been found that highly satisfactory operation is obtainable when the height of the cathode protrusions is greater than about 20% and up to approximately 70% of the cathode-to-anode interelectrode spacing. With these protrusion heights the roots or valleys between the protrusions are removed from the path of returning electrons so that such electrons traverse a small loop in the magnetic field and reenter the interaction space for contributing to the RF power. Tubes incorporating the 70% protrusion height have been operated with particu larly satisfactory results. Additionally, a protrusion width equaling approximately the spacing between adjacent protrusions is preferred. However, a width range of between about 25 to about 60% of the periodic cathode pitch, or distance between corresponding points along the circumference of the cathode, is believed suitable. Further, I prefer forming the cathode protrusions so as to have at least slightly curved edges to avoid the presence of extremely high electrical fields thereat which can cause rapid deterioration of the emissive material and thus result in changes in emissivity during extended periods of operation.

It is to be understood from the foregoing that my im- 18 proved structure involves generally the provision of a cathode structure including periodically emissive sections or regions and a predetermined field distribution between the emissive sections and cavity resonator segments of an anode structure such as to provide concentrated DC. electric field regions at the periodic emissive sections and substantially field-free regions therebetween. This structure enhances effects of the favorable electrons in formation of rotating spokes of bunched electrons and contributes positively to the radio frequency power output. It

minimizes back heating caused by the effects of unfavorable electrons and variations in back heating caused by load variations. It also enables predetermined adjustment of the cathode temperature resulting from back eating. The reduction of back heating enables operation of the magnetron at a substantially higher power level for a given operating temperature. The reduced tendency toward back heating allows the use of a smaller diameter cathode which, in turn, allows the use of a smaller inner diameter anode. In this manner it is possible to reduce the ratio of anode diameter to the number of anode cavities which, as noted above, provides for increased operating efficiency and power output. Additionally, substantially uniform cathode temperature generally insensitive to load variations is obtainable which affords greater uniformity of power output. Further, my improved structure increases or maximizes the tolerable load that may be placed on a magnetron.

In a typical illustrative example of the magnetron device 1 designed to operate at a frequency of 915 megacycles, the wall member 7 has an external diameter of about 3 inches and a vertical dimension of about 2% inches as viewed in FIG. 1 and an internal diameter of about 2% inches; each of the vanes 20 has a radial extent of about 1.04 inches and a vertical extent as viewed in FIG. 1 of about inch at the inner end portion 21 and at the outer portion 22, whereby the diameter of the generally cylindrical space between the inner end portions 21 is about 0.67 inch; the cathode 6 has a diameter of about 0.58 inch and a vertical extent of about 0.75 inch; the straps 33 have an external diameter of about 1.47 inches and the straps 32 have an external diameter of about 1.15 inches; the disk 61 has an external diameter of about 1.2 inches. the rods have a length between the strap 32 and the disk 61 or about /2 inch and the enlarged portions of the central conductor 62 disposed between the disk 61 and the associated end cap 10 has a length of about 0.45 inch; and the ring 66 has an external diameter of about 1.18 inches. All of the other parts of the magnetron device 1 and the magnetic circuit 2 are correspondingly dimensioned as illustrated in the several figures of the drawings.

It is to be understood still further that while the various advantageous features of my invention have been discussed with reference to electronic cooking applications, such features are clearly applicable to crossed-field devices aclauted for other applications.

While I have shown and described specific embodiments of my invention, I do not desire my invention to be limited to the particular forms shown and described and I intend,

' by the appended claims. to cover all modifications within the spirit and scope of my invention.

What is claimed is:

1. A low voltage crossed-field electric discharge device comprising an anode structure defining an axially extending generally cylindrical space therein and a plurality of resonator cavities opening into said space, a generally cylindrical cathode disposed in said space, the ratio between the outer diameter of said cathode and the inner diameter of said anode structure being N(1.50 to 2.5) NWT?) wherein N is equal to the number of resonator cavities in said anode structure and is a number between 16 and 36,

and means for establishing a unidirectional magnetic field extending axially through said space, said device being characterized by stability during the operation thereof when the applied voltage between said cathode and said anode structure is in the general range from about 400 volts DC. to about 1,000 volts DC.

2. The low voltage crossed-field electric discharge device set forth in claim 1, wherein the ratio between the outer diameter of said cavity and the inner diameter of said anode structure is 3. A low voltage crossed-field electric discharge device comprising an envelope, an anode structure in said envelope and including a plurality of radially extending anode segments defining an axially extending generally cylindrical space in said anode structure and a plurality of resonator cavities opening into said space, a generally cylindrical cathode disposed in said space, means for establishing a unidirectional magnetic field extending axially through said space, and an output circuit comprising an outer annular conductor and an inner conductor disposed within said outer conductor, the inner end of said outer conductor extending through said envelope and being connected to one end of said anode structure adjacent to the outer portion thereof, the inner end of said inner conductor extending through said envelope and being conductively connected to a plurality of pairs of angularly spaced portions of said anode structure adjacent to the inner periphery thereof, said output circuit having a low pass filter characteristic providing a low resistance parallel current path at the pi mode frequency of operation of said device and for attenuating internally of said envelope radiations having frequencies higher than the pi mode ferquency of operation of said device.

4. A low voltage crossed-field electric discharge device comprising an envelope, an anode structure in said envelope and including a plurality of radially extending aniode segments defining an axially extending generally cylindrical space in said anode structure and a plurality of resonator cavities opening into said space, a generally cylindrical cathode disposed in said space, means for establishing a unidirectional magnetic field extending axially through said space, and an output circuit comprising an outer annular conductor and an inner conductor disposed within said outer conductor, the inner end of said outer conductor cxtending through said envelope and being connected to one end of said anode structure adjacent to the outer portion thereof, the inner end of said inner conductor extending through said envelope and being conductively connected to a plurality of pairs of angularly spaced portions of said anode structure adjacent to the inner periphery thereof, wherein each adjacent pair of conductive connections between said inner conductor and said anode structure is separated by a plurality of resonator cavities such as specified by the mathematical representation where P is equal to the number of conductive connections, N is equal to the number of resonator cavities in said anode structure, and M is any integer which i greater than 1 but less than N/ 2 and which will make P a whole number, said output circuit having a low pass filter characteristic providing a low resistance parallel current path at the pi mode frequency of operation of said device and for attenuating internally of said envelope radiations having frequencies higher than the pi mode frequency of operation of said device.

5. The low voltage crossed-field electric discharge device set forth in claim 4, wherein each adjacent pair of conductive connections between said inner conductor and 2@ said anode structure is separated by a plurality of resonator cavities equal to a number selected fnom the group Of numbers consisting of 6 and 8 and 10.

6. The low voltage crossed-field electric discharge d vice set forth in claim 3, and further comprising a concentric seal disposed between said inner conductor and said envelope and providing electrical insulation therebetween, said seal being located at a point disposed A electrical wavelength away from said conductive connections between said inner conductor and said anode strueture at the normal operating frequency of said device.-

'7. A low voltage crossed-field electric discharge device comprising an envelope, an anode structure in said envelope and including a plurality of pairs of radially extending anode segments defining an axially extending space in said anode structure and a plurality of pairs of resonator cavities opening into said space, a generally cylindrical cathode disposed in said space, means for establishing a unidirectional magnetic field extending axially through said space, an output circuit comprising an outer annular conductor and an inner conductor disposed within said outer conductor, the inner end of said outer conductor extending through said envelope and being connected to one end of said anode structure, the inner end of said inner conductor extending through said envelope and coupled to selected ones of said cavities, said output circuit having a low pass filter characteristic providing a low resistance parallel current path at the pi mode frequency of operation of said device and for attenuating internally of said envelope radiations having frequencies higher than the pi mode frequency of operation of said device, and a conductive ring supported in spaced relation to the other end of said anode structure providing a compensating capacitance for said output cit cuit and effective at the pi mode frequency of operation of said device to compensate for any electrical unbalance in said device introduced therein'to by said output circuit,

8. The low voltage crossed-field electri dischafge de--- vice set forth in claim 7, and further comprising a first strapping element conductively intercdnnecting alternate anode segments on said one end ofsaid antide structure, and a second strapping element conductively interconnectingthe remaining anode segments on the Other end of said anode structure, said inner conductor being directly conductively connected to said first strapping element.

9. The low voltage crossed-field electric discharge device set forth in claim 7, and further comprising two pairs of concentric conductive rings respectively arranged on the opposite ends of Said anode structure, each of said rings nterconnecting an alternate set of anode segments, the 1nner ring on one end of said anode structure intercom necting the same alternate set of anode segments as the outer ring on the other end of said anode structure, said 1nner conductor being directly conductively connected to the inner one of said rings on said one end of said anode r structure.

d0. A low voltage crossed-field electric discharge device comprising an envelope, an anode structure in said envelope and including a plurality of radially extending anode segments defining an axially extending generally cylindrical space in said anode structure and a plurality of resonator cavities opening into said space, a generally cylmdrical cathode disposed in said space, the ratio between the outer diameter lOf said cathode and the inner diameter of said anode structure being N+ (1.50 to 2153 where N is equal to the number of resonator cavities in said anode structure and is a number between 16 and 36, means for establishing a unidirectional magnetic field extending axially through said space, said device being characterized by stability during the operation thereof when the applied voltage between said cathode and said anode structure is in the general range from about 400 volts DC. to about 1,000 volts D.C., and an output circuit comprising an outer annular conductor and an inner conductor disposed within said outer conductor, the inner end of said outer conductor extending through said envelope and being connected to one end of said anode structure adjacent to the outer portion thereof, the inner end of said inner conductor extending through said envelope and being conductively connected to a plurality of pairs of angularly spaced portions of said anode structure adjacent to the inner periphery thereof, said output circuit having a low pass filter characteristic providing a low resistance parallel current path at the pi mode frequency of operation of said device and for attenuating internally of said envelope radiations having frequencies higher than the pi mode frequency of operation of said device.

11. A low voltage crossed-field electric discharge device comprising an envelope, an anode structure in said envelope and including a plurality of radially extending anode segments defining an axially extending generally cylindrical space in said anode structure and a plurality of resonator cavities opening into said space, a generally cylindrical cathode disposed in said space, the ratio between the outer diameter of said cathode and the inner diameter of said anode structure being N(1.50 to 2.5) N+(1.5O to 2.5

where N is equal to the number of resonator cavities in said anode structure and is a number between 16 and 36, means for establishing a unidirectional magnetic field extending axially through said space, and an output circuit comprising an outer annular conductor and an inner conductor disposed within said outer conductor, the inner end of said outer conductor extending through said envelope and being connected to one end of said anode structure adjacent to the outer portion thereof, the inner end of said inner conductor extending through said envelope and being conductively connected to a plurality of pairs of angularly spaced portions of said anode structure adjacent to the inner periphery thereof, wherein each adjacent pair of conductive connections between said inner conductor and said anode structure is separated by a plurality of resonator cavities such as specified by the mathematical representation AT nr where P is equal to the number of conductive connections, N is equal to the number of resonator cavities in said anode structure, and M is any integer which is greater than 1 but less than N/ 2 and which will make P a whole number, said output circuit having a low pass filter characteristic providing a low resistance parallel current path at the pi mode frequency of operation of said device and for attenuating internally of said envelope radiations having frequencies higher than the pi mode frequency of operation of said device.

12. The low voltage crossed-field electric discharge device set forth in claim 10, and further comprising a concentric seal disposed between said inner conductor and said envelope and providing electrical insulation therebetween, said seal being located at a point disposed electrical wavelength away from said conductive connections between said inner conductor and said anode structure at the normal operating frequency of said device.

13. A low voltage crossed-field electric discharge de vice comprising an envelope, an anode structure in said envelope and including a plurality of pairs of radially extending anode segments defining an axially extending generally cylindrical space in said anode structure and a plurality of pairs of resonator cavities opening into said space, a generally cylindrical cathode disposed in said space, the ratio between the outer dia-rrieter' of said oath ode and the inner diameter of said anode structure being N(1.50 to 2.5) N+(1.50 to 2.5

where N is equal to the number of resonator cavities in said anode structure and is a number between 16 and 36, means for establishing a unidirectional magnetic field extending axially through said space, an output circuit comprising an outer annular conductor and an inner conductor disposed within said outer conductor, the inner end of said outer conductor extending through said envelope and being connected to one end of said anode structure, the inner end of said inner conductor extending through said envelope and coupled to selected ones of said cavities, said output circuit having a low pass filter characteristic providing a low resistance parallel current path at the pi mode frequency of operation of said device and for attenuating internally of said envelope radiations having frequencies higher than the pi mode frequency of operation of said device, and a conductive ring supported in spaced relation to the other end of said anode structure providing a compensating capacitance for said output circuit and effective at the pi mode frequency of operation of said device to compensate for any electrical unbalance in said device introduced thereinto by said output circuit.

14. A low voltage crossed-field electric discharge device comprising an anode structure including a plurality of radially inwardly extending anode segments defining an axially extending generally cylindrical space, a generally cylindrical cathode structure disposed in said space and cooperating with said anode structure to provide an annular interaction space, and means for establishing a unidirectional magnetic field extending axially through said interaction space, said cathode structure including a plurality of circumferentially spaced apart radially outwardly protruding emissive sections thereon equal in number to said anode segments, wherein the centerline of each of said protruding emissive sections is angularly displaced relative to the centerline of the adjacent anode segment in accordance with the following mathematical representation:

where R is the optimum distance of an angular displacement of the centerline of each emissive protruding section with respect to the adjacent anode section, y is the minimum interelectrode spacing between said protruding emissive sections and said anode segments, and X is the dis tance between the inner ends of adjacent anode segments.

15. A low voltage crossed-field electric discharge device comprising an anode structure including a plurality of radially inwardly extending anode segments defining an axially extending generally cylindrical space, a generally cylindrical cathode structure disposed in said space and cooperating with said anode structure to provide an annular interaction space, and means for establishing a unidirectional magnetic field extending axially through said interaction space, the ratio between the outer diameter of said cathode and the inner diameter of said anode structure being N(l.50 to 2.5) N+(l.50 to 2.5)

where N is equal to the number of resonator cavities in said anode structure and is a number between 16 and 36, said cathode structure including a plurality of circumferentially spaced apart radially outwardly protruding emissive sections thereon equal in number to said anode segments, the centerline of each of said protruding emissive sections being angularly displaced relative to the centerline of the adjacent anode segment a distance equal to between about 0% and about 45% of the spacing between the inner ends of said anode segments.

where N is equal to the number of resonator cavities in said anode structure and is a number between 16 and 36, said cathode structure including a plurality of circumferentially spaced apart radially outwardly protruding emissive sections thereon equal in number to said anode segments, wherein the centerline of each of said protruding emissive sections is angularly displaced relative to the centerline of the adjacent anode segment in accordance with the following mathematical representation:

2 2.5X Where R is the optimum distance of an angular displacement of the centerline of each emissive protruding section with respect to the adjacent anode section, y is the minimum interelectrode spacing between said protruding emissive sections and said anode segments, and X is the distance between the inner ends of adjacent anode segments.

17. A magnetron device comprising an envelope including a cylindrical wall section and conductive end caps closing the ends of said cylindrical section, an anode structure in said envelope including a plurality of radially extending anode segments defining an axially extending space in said anode structure and a plurality of resonator cavities opening into said space, said anode structure being conductively connected to said end wall, a generally cylindrical cathode disposed in said space, the ratio of the radius of the outer diameter of said cathode to the radius of the inner diameter of said anode being 1 v(1.50 to 2.5) N+(1.50 to 2.5)

wherein N equals the number of resonator cavities in said anode structure, a pair of concentric conductive rings onv each side of said anode structure, each said rings interconnecting an alternate set of anode segments with the inner ring on one side interconnecting the same alternate set of anode segments as the outer ring on the opposite side of said anode structure, a magnetic pole plate extending across said envelope in an end space between each side of said anode structure and the corresponding ends of said envelope, an energy output circuit comprising one of said end caps, 21 separate center conductor extending concentrically through and electrically insulated from said one end cap, means including several discrete conductors extending between said center conductor and angularly spaced points of one of said strapping elements, said discrete conductors extending in electrically spaced relation through apertures in the corresponding pole plate, and the points of connection to said strapping element being separated by a plurality of resonator cavities such as to satisfy the mathematical representation:

where P is the number of discrete points of connection, N is the number of resonator cavities in the anode structure and M is any integer which is greater than 1 but less than N 2 and which make P a whole number, and means comprising a conductive ring supported by the other of said pole plates in predetermined spaced relation to the strapping element on said opposite side of said anode structure and which is of opposite polarity to the strapping element to which the output connections are made, the said lastmentioned means introducing a predetermined capacity compensating for any unbalance introduced into the operation of said device by the unusually close interelectrode spacing between said anode and said cathode or by said output circuit.

18. The low voltage crossed-field electric discharge device set forth in claim 1, wherein said anode structure includes a plurality of radially inwardly extending anode segments defining said resonator cavities, said cathode structure includes a plurality of circumferentially spaced apart radially outwardly protruding emissive sections thereon equal in number to said anode segments, the centerline of each of said protruding emissive sections being angularly displaced relative to the centerline of the adjacent anode segment a distance equal to between about 0% and about 45% of the spacing between the inner ends of said anode segments.

19. The low voltage crossed-field electric discharge device set forth in claim 1, wherein said anode structure includes a plurality of radially inwardly extending anode segments defining said resonator cavities, said cathode structure includes a plurality of circumferentially spaced apart radially outwardly protruding emissive sections thereon equal in number to said anode segments, the centerline of each of said protruding emissive sections is angularly displaced relative to the centerline of the adjacent anode segment a distance equal to between about 15% and about 35% of the spacing between the inner ends of said anode segments.

20. The low volt-age crossed-field electric discharge device set forth in claim 1, wherein said anode structure includes a plurality of radially inwardly extending anode segments defining said resonator cavities, said cathode structure includes a plurality of circumferentially spaced apart radially outwardly protruding emissive sections thereon equal in number to said anode segments, the centerline of each of said protruding emissive sections is angularly displaced relative to the centerline of the adjacent anode segment a distance equal to between about 0% and about 45% of the spacing between the inner ends of said anode segments and on the downstream side of the adjacent anode segment relative to the direction of normal electron flow in said device.

References Cited by the Examiner UNITED STATES PATENTS 2,542,899 2/1951 Brown 315-39 2,592,206 4/1952 Sproull 315-3951 X 2,597,506 4/1952 Ludi 315-3967 X 2,611,110 9/1952 Powers 315-3961 2,721,294 10/1955 Shelton 315-3953 2,784,346 3/1957 Dodds 315-3963 2,992,362 7/1961 Boyd 315-3971 X 3,121,822 2/1964 Boyd 315-3951 X HERMAN KARL SAALBACH, Primary Examiner.

ELI LIEBERMAN, S. CHATMON, In,

Assistant Examiners. 

1. A LOW VOLTAGE CROSSED-FIELD ELECTRIC DISCHARGE DEVICE COMPRISING AN ANODE STRUCTURE DEFINING AN AXIALLY EXTENDING GENERALLY CYLINDRICAL SPACE THEREIN AND A PLURALITY OF RESONATOR CAVITIES OPENING INTO SAID SPACE, A GENERALLY CYLINDRICAL CATHODE DISPOSED IN SAID SPACE, THE RATIO BETWEEN THE OUTER DIAMETER OF SAID CATHODE AND THE INNER DIAMETER OF SAID ANODE STRUCTURE BEING 